The production of bioethanol from the lignocellulose that constitutes the wall of plants is today an important issue on account of:                the availability of lignocellulosic plant raw material (cereal straw, sugar cane bagasse, wood, etc.);        the increase in the cost of fossil energies and especially oil and the necessity that follows therefrom to have substitutes for some of the refined petroleum products, in particular for their use as fuels; and        the recent and very high increase in the cost of cereals due, for the main part, to the competition between the s conventional food sector and the production of bioethanol from grains.        
Lignocellulose is a generic term that denotes the natural composite material that gives shape and structure to plants. It is the combination of three natural biopolymers, which are:                cellulose, which is a stereoregular polysaccharide resulting from the polymerization of D-glucose to β-1,4-glucose;        lignins, which are polyphenols resulting from the polymerization of phenolic allyl alcohols; and        hemicelluloses, which are polysaccharides resulting from is the polymerization of sugars having 5 (five) carbon atoms such as xylose and arabinose and/or having 6 (six) carbon atoms such as glucose and mannose.        
The proportion and the distribution of the various sugars depends on the plant in question.
Thus, the hemicelluloses of annual plants and of hardwoods have, in a very large proportion, xylose as the main monomer, whereas the main monomer of hemicelluloses of softwoods is mannose.
These three macromolecules are arranged as supramolecular and anatomical structures that are much more complex than the structure of starch granule or of free sucrose, which explains why the utilization of lignocelluloses in the form of bioethanol raises specific problems especially on an industrial scale.
The cellulose and hemicelluloses from these plants are the essential part of terrestrial biomass. These two biopolymers therefore constitute an enormous reserve of fermentable sugars on a worldwide scale, which bears no comparison with the glucose derived from the fermentation of starch, of cereal grains or of potatoes.
Depending on the initial plant raw material, the process for producing bioethanol generally comprises three large main sets of s operations, that is to say, consecutively A) the preparation of a wort, then B) the fermentation of the wort for the purpose of obtaining a fermented wort, then D) the distillation of the fermented wort for the purpose of producing bioethanol.
To these three large sets of operations, it is possible to add a fourth general set of operations E) that consists of the various treatments of the co-products resulting from each of these three main sets of operations.
All the operations A) for preparing the wort aim to prepare a paste or a liquor comprising the plant raw material capable of is being fermented, that is to say an aqueous solution of sugars that can be fermented by yeasts, by aiming to obtain the highest possible concentration so as to reduce the capacities of the equipment necessary for preparing the wort and for other subsequent operations. In the case of production from lignocellulosic resources, the conversion of the cellulose and hemicelluloses to ethanol requires a prior incontrovertible “step” of depolymerization to sugar monomers, followed by their fermentation.
Since the 1980's, it has thus appeared that solving problems specific to the production from lignocellulosic resources imposed the optimal functioning of the following steps. LIGNOCELLULOSES→Pretreatment→CELLULOSE+LIGNINS→(Enzymatic) hydrolysis→GLUCOSE+LIGNINS→Fermentation→Distillation→ETHANOL/BIOETHANOL
Although the fermentation operation has been known since the first conversion of a sugary liquor to an alcoholic drink and is therefore carried out annually on tens of millions of metric tons, the same is not true for the pretreatment of cellulose which, to date, has no profitable industrial application in the world.
For more than half a century, numerous studies have been carried out that relate to pretreating plant material so as to make s the cellulose able to be hydrolyzed to glucose under industrially acceptable conditions.
None of the processes studied have to date actually succeeded on an industrial scale, despite the tremendous means that have been dedicated thereto.
These known pretreatments generally proceed via a first dissolution in water of some of the hemicelluloses in the form of monomers, oligomers and polymers in acid or basic medium.
The lignocellulose is then treated so as to obtain monosaccharides, oligosaccharides, or even polysaccharides that is can be easily fermented by pretreatments via:                acid hydrolysis of the polysaccharides under “harsh” conditions at high temperatures (120 to 250° C.) and under high pressures with concentrations of acids that may range up to 12 wt %;        steam explosion at high pressures (1 to 3×106 Pa) and high temperatures (190 to 220° C.);        addition of an organic solvent that facilitates the destructuring of the plant in question; and        enzymatic hydrolysis followed by fermentation of the hydrolyzate, combined with an ultrasound treatment.        
All of these known pretreatments certainly facilitate the conversion of cellulose to alcohol, but have the major drawback of producing a polysaccharide-contaminated ligneous residue that is then difficult to utilize other than by incineration.
These lignins furthermore have the drawback of interfering with the action of enzymes during the hydrolysis step that follows the pretreatment, especially on account of the presence of lignins and of furfural present in most acid prehydrolyses.
Furthermore, this type of pretreatment results in a significant cost, which is prohibitive on an industrial scale, especially on account of the investment in equipment and of the need to use steam.